This invention relates to a cutting element for an earth boring bit, and more particularly to a cutting element having improved stress distribution between a substrate and a superabrasive cutting compact.
Cutting compacts used as cutting elements in rotary drill bit construction typically comprise a layer of synthetic diamonds, conventionally in the form of polycrystalline diamond. Polycrystalline diamond compact cutting elements, commonly known as PDC""s, have been commercially available for many years. Although there has been some use of PDC""s as a self-supporting layer, the more recent utilization of the polycrystalline diamond compact is in the form a substantially planar diamond layer bonded during formation to a supporting substrate.
Previous uses of the PDC have demonstrated that these compacts are resistant to abrasion and erosion, although there are several identified disadvantages. polycrystalline diamond and tungsten carbide, main components of a PDC, are brittle materials that easily fracture on impact. Another recognized disadvantage of the PDC is the different coefficient of thermal expansion between tungsten carbide and polycrystalline diamond. As a result of this different coefficient of thermal expansion, residual stresses have been identified and are a result of the greater contraction of tungsten carbide over synthetic diamond during a cooling phase. This thermally induced stress between the various components of the PDC results in a reduction in the bond strength between the two components.
During use of a PDC, it has been observed that impact forces relieve the residual sheer stresses resulting in fractures in the compact. The result of this fracturing is that the diamond layer spalls and/or delaminates resulting in a separation and loss of the diamond layer resulting in a failure of the PDC.
Heretofore, a common problem with cutting elements of superabrasive compacts bonded to a substrate is spalling and delamination of the superabrasive layer from the substrate. Spalling and delamination result from subjecting the cutting element to extreme temperatures and heavy stress load fluctuation when a drill bit is in use down a bore hole. During operation at extremely high temperatures, thermally induced stresses have been identified at the interface between the superabrasive cutting compact and the supporting substrate, the magnitude of the stresses being a function of the disparity in the thermal expansion coefficients of adjacent materials.
There are numerous patents granted directed to various attempts made to limit the effects of thermal induced stress by modifying the geometry of the interface between the diamond and the tungsten carbide. As illustrated by a review of earlier issued patents, the interface modification many times replaces a planar interface with an irregular, non-planar interface geometry. Many of the early attempts to solve the stress related disadvantages of the PDC claim as an advantage the redistribution of residual stresses. A redistribution of residual stresses does allow an increase in the diamond thickness thereby resulting in an increase in bit life. The non-linear planar interface between the diamond and the tungsten carbide substrate results in an enhanced mechanical interlocking and improved stability and performance of the cutting element and therefore translates into longer bit life.
While cutting elements utilizing the superabrasive cutting compacts employed in rotary drill bits for earth boring have achieved major advances in obtainable rate of penetration at economically viable costs, the interface between the superabrasive cutting compact and the supporting substrate leaves something to be desired. As a result, considerable activity has been directed toward attempts to improve the bond between the superabrasive cutting compact and the supporting substrate by configuring the rear face of the cutting compact so as to provide a degree of mechanical interlocking between the cutting compact and the supporting substrate. Several United States patents directed to solutions for this problem have been granted including U.S. Pat. Nos. 5,617,928; 4,784,023 and 5,351,772, to identify only a few, describe various techniques for improving the bond between the superabrasive cutting compact and the supporting substrate.
The present invention relates to improvements in the interface between the superabrasive cutting compact and the supporting substrate.
According to the invention there is provided a superabrasive cutting compact integral with a substrate to form a cutting element. The cutting compact comprises a pattern of radially extending ribs where the ribs extend from in proximity to a peripheral edge of the compact. A circular shaped pattern of circular ribs radially spaced from the peripheral edge of the compact intersect the pattern of radially extending ribs. The substrate supporting the cutting compact comprises a pattern of radially extending grooves for mating with the pattern of radially extending ribs. The substrate also comprises a pattern of circular grooves for mating with the pattern of circular ribs of the cutting compact.
The ribs of the cutting compact and the grooves of the substrate have an expanding width dimension from one circumferential segment of a peripheral edge of the cutting element to the opposite circumferential segment. Further, in accordance with this alternate embodiment, the circular pattern of ribs and the circular pattern of grooves comprise segments of concentric circles spaced from the peripheral edge. The ribs and grooves can have different widths and depths.
In another embodiment of the invention, the substrate surface comprises a convex surface and the mating surface of the cutting compact comprises a concave surface.
Alternatively, the cutting element comprises a cylinder with the cutting compact fixed perpendicular to the axis of the cylinder or the superabrasive cutting compact is affixed directly to a stud insert for use with a rotary drill bit.
A technical advantage of the present invention is a cutting element having improved stress distribution between a superabrasive material cutting compact and a supporting substrate resulting in enhanced performance of a cutting element as part of a rotary drill bit. An additional technical advantage of the present invention is providing an improved bond between the superabrasive material cutting compact and the substrate surface affixed thereto.